Electret covered with an insulated film and an electret condenser having the electret

ABSTRACT

A silicon nitride film ( 103 ) and a silicon nitride film ( 106 ) are formed to cover a charged silicon oxide film ( 105 ) serving as an electret.

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/JP2004/016835, filed Nov. 12, 2004,which in turn claims the benefit of Japanese Application No.2003-390554, filed Nov. 20, 2003, Japanese Application No. 2004-019616,filed Jan. 28, 2004, and Japanese Application No. 2004-253894, filedSep. 1, 2004, the disclosures of which Applications are incorporated byreference herein in their entirety.

TECHNICAL FIELD

The present invention relates to an electret condenser having avibrating electrode and a fixed electrode and, more particularly, to anelectret condenser formed by using a MEMS (Micro Electro MechanicalSystems) technology.

BACKGROUND ART

Organic high-molecular polymers such as FEP (Copolymer ofTetrafluoroethylene (TFE) and Hexafluoropropylene (HFP)) materials havebeen used conventionally for electret elements which are dielectricmaterials each having a permanent electric polarization and applied todevices such as a condenser microphone. However, since these materialsare inferior in thermal resistance, the problem has been encounteredthat they are difficult to use as elements for reflow when mounted onsubstrates.

As a solution to the problem, an electret using a silicon oxide film asshown in Patent Document 1, instead of an organic high-molecularpolymer, has been proposed in recent years to provide a thinner-film andsmaller-size electret by using a microfabrication technology.

Specifically, the technology shown in Patent Document 1 deposits asilicon oxide film on a surface of a base, sets a gas atmospherecontaining oxygen and containing no moisture in a deposition chamberwithout releasing the chamber to an ambient atmosphere, performs athermal process at 200° C. to 400° C. with respect to the silicon oxidefilm in the atmosphere, and then performs a charging process withrespect to the silicon oxide film.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-33241

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, an electret has the problem of losing charge upon contact witha liquid. For example, when electretized FEP is immersed in ethanol, acharge in the FEP is significantly reduced, though the charge does notbecome zero. According to an experiment conducted by the presentinventors, when FEP (specifically, FEP formed on a stainless steelsubstrate to have a thickness of 12.5 μm) of which the surface potentialindicating an amount of charge was 300 V was immersed in ethanol, thesurface potential was reduced to the order of several volts. It is to benoted that this phenomenon similarly occurs even when the electret isimmersed not only in ethanol but also in another organic solvent orwater. In terms of the material also, the phenomenon is not peculiar toFEP but similarly occurs in a general electret material such as asilicon oxide film.

In view of the foregoing, it is therefore an object of the presentinvention to provide an element to which an electret condenser has beenapplied and which has a structure excellent in moisture resistance, suchas an ECM (electret condenser microphone). Another object of the presentinvention is to provide a small-size ECM which does not require a chargesupply circuit by producing an ECM composed of an electret having apermanent charge by using a MEMS technology.

Means for Solving the Problem

To attain the objects described above, an electret according to thepresent invention comprises a charged silicon oxide film and aninsulating film formed to cover the silicon oxide film.

A first electret condenser according to the present invention comprises:a first electrode formed with through holes; a second electrode disposedwith an air gap interposed between itself and the first electrode; andan electret composed of a charged silicon oxide film formed on a surfaceof the second electrode which is opposing the first electrode, whereinan insulating film is formed to cover the silicon oxide film.

A second electret condenser according to the present inventioncomprises: a fixed film having a first electrode and formed with firstthrough holes; a second electrode disposed with an air gap interposedbetween itself and the fixed film; and an electret composed of a chargedsilicon oxide film formed on a surface of the second electrode which isopposing the fixed film, wherein an insulating film is formed to coverthe silicon oxide film.

A third electric condenser according to the present invention comprises:a semiconductor substrate having a region removed to leave a peripheralportion thereof; and a vibrating film formed on the semiconductorsubstrate to cover the region, wherein the vibrating film has amultilayer structure composed of an electret, an electrode film, a firstinsulating film, and a second insulating film and said electret iscovered with each of the first insulating film and the second insulatingfilm.

The electret and electret condenser according to the present inventionallows protection of the surfaces of the charged silicon oxide film,i.e., the upper, lower, and side surfaces thereof with the insulatingfilm. Specifically, by covering the silicon oxide film which showsremarkable absorption of atmospheric moisture or the like with theinsulating film to prevent the surfaces thereof from being exposed to anambient atmosphere, it becomes possible to suppress a reduction in theamount of charge in the charged (electretized) silicon oxide film. Thisallows an improvement in the reliability of the electret.

In the electret and electret condenser according to the presentinvention, the insulating film need not directly cover a surface of thecharged silicon oxide film (electret), e.g., the upper or lower surfacethereof. For example, an electrode may also be interposed between thelower surface of the silicon oxide film and the insulating film.

In the electret and electric condenser according to the presentinvention, the insulating film covering the charged silicon oxide film(electret) preferably has a higher moisture resistance than the siliconoxide film. More specifically the moisture resistance (resistance tocharge loss in a given humidity state such as, e.g., a moistureresistance test) of the silicon oxide film covered with the insulatingfilm is higher than that of the silicon oxide film uncovered with theinsulating film. As the insulating film having a higher moistureresistance than the silicon oxide film, there can be used, e.g., asilicon nitride film.

Effect of the Invention

In accordance with the present invention, there can be provided anelement to which an electret condenser has been applied and which has anelectret structure excellent in moisture resistance, such as an ECM. Byproducing such an ECM by using a MEMS technology, it becomes possible toprovide a small-size ECM which does not require a charge supply circuit.Thus, the present invention renders it possible to implement ahigh-reliability, small-size, and high-performance microphone. Inaddition, it also becomes possible to widely supply various practicaldevices each equipped with the microphone to a society.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) are structural views of an ECM according to anembodiment of the present invention, of which 1(a) is a plan view of theECM and FIG. 1( b) is a cross-sectional view of the ECM;

FIG. 2 is a circuit block diagram of the ECM according to theembodiment;

FIG. 3 is a cross-sectional view of an electret condenser composing theECM according to the embodiment;

FIG. 4 is a plan view of the lower electrode of the electret condensercomposing the ECM according to the embodiment and extraction wiringthereof; and

FIG. 5 is a plan view of a silicon nitride film in the fixed film of theelectret condenser composing the ECM according to the embodiment.

DESCRIPTION OF NUMERALS

18 Microphone Portion

19 SMD

20 FET Portion

21 Printed Board

22 Case for ECM

23 Internal Circuit of ECM

24 Output Terminal

25 Output Terminal

26 External Terminal

27 External Terminal

28 Terminal

29 Terminal

30 Terminal

101 Semiconductor Substrate

102 Silicon Oxide Film

103 Silicon Nitride Film

104 Lower Electrode

105 Silicon Oxide Film

106 Silicon Nitride Film

107 Leak Hole

108 Silicon Oxide Film

109 Air Gap

110 Fixed Film

111 Acoustic Hole

112 Vibrating Film

113 Membrane Region

114 Silicon Nitride Film

115 Extraction Wiring

116 Opening

117 Opening

118 Conductive Film

119 Silicon Nitride Film

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment

Referring to the drawings, an electret condenser according to anembodiment of the present invention will be described by using the casewhere it is applied to an ECM as an example.

A description will be given first to the ECM as an element to which theelectret condenser according to the present embodiment has been applied.

FIGS. 1( a) and 1(b) are structural views of the ECM according to thepresent embodiment, of which FIG. 1( a) is a plan view of the ECM andFIG. 1( b) is a cross-sectional view of the ECM.

As shown in FIGS. 1( a) and 1(b), the ECM according to the presentembodiment is comprised of: a microphone portion 18; a SMD (SurfaceMounted Device) 19 such as a condenser; and a FET (Field EffectTransistor) portion 20 which are mounted on a printed board 21. As shownin FIG. 1( b), the printed board 21 with the microphone portion 18, theSMD 19, and the FET portion 20 mounted thereon is protected by a case22, though the depiction thereof is omitted in FIG. 1( a).

FIG. 2 is a circuit block diagram of the ECM according to the presentembodiment.

As shown in FIG. 2, the internal circuit 23 of the ECM according to thepresent embodiment is comprised of: the microphone portion 18 composedof an electret condenser according to the present embodiment, which willbe described later; the SMD 19; and the FET portion 20. From the outputterminals 24 and 25 of the internal circuit 23, respective signals areoutputted to external terminals 26 and 27. During actual operation, whena signal having a voltage of, e.g., about 2 V is inputted from theterminal 28 which is connected to the external terminal 26 via aresistor, a signal having an AC voltage of, e.g., several tens ofmicrovolts is outputted to the terminal 29 which is connected to theexternal terminal 26 via a condenser. Each of the external terminal 27and the terminal 30 connected thereto is connected to the outputterminal 25 as the GND terminal in the ECM internal circuit 23.

A description will be given herein below to the electret condenseraccording to the present embodiment. FIG. 3 is a cross-sectional view ofthe electret condenser according to the present embodiment.

As shown in FIG. 3, the electret condenser according to the presentembodiment has a parallel-plate condenser structure which uses, aselectrodes, a vibrating film 112 formed above a semiconductor substrate101 having a region (hereinafter referred to as a membrane region 113)removed to leave the peripheral portion thereof such that the membraneregion 113 is covered with the vibrating film 112 and a fixed film 110disposed with an air gap 109 interposed between itself and the vibratingfilm 112. The vibrating film 112 has a lower electrode 104, while thefixed film 110 has a conductive film (upper electrode) 118.

In the electret condenser according to the present embodiment, when thevibrating film 112 receives a sound pressure from above through aplurality of acoustic holes 111 provided in the fixed film 110 and theair gap 109, the vibrating film 112 mechanically vibrates upward anddownward in response to the sound pressure. When the vibrating film 112vibrates, the distance (distance between the electrodes) between thevibrating film 112 (i.e., the lower electrode 104) and the fixed film110 changes so that the capacitance (C) of the condenser changesaccordingly. Since a charge (Q) accumulated in the condenser isconstant, the change in the capacitance (C) of the condenser causes achange in the voltage (V) between the lower electrode 104 and the fixedfilm 110. The reason for this is that a condition given by the followingnumerical expression should be physically satisfied.Q=C·V  (1)

Because the lower electrode 104 is electrically connected to the gate ofthe FET portion 20 of FIG. 2, the gate potential of the FET portion 20changes with the vibration of the vibrating film 112. The change in thegate potential of the FET portion 20 is outputted as a voltage change tothe external output terminal 29.

A detailed structure of the electret condenser according to the presentembodiment is as follows.

As shown in FIG. 3, a silicon oxide film 102 is formed on thesemiconductor substrate 101 on which the electret condenser according tothe present embodiment is mounted and the membrane region 113 is formedby partially removing the semiconductor substrate 101 and the siliconoxide film 102 such that the respective peripheral portions thereofremain. Thus, the membrane region 113 is a region formed by partiallyremoving the semiconductor substrate 101 such that the peripheralportion thereof remains to allow the vibrating film 112 to vibrate onreceiving a pressure from the outside.

On the silicon oxide film 102, the silicon nitride film 103 is formed tocover the membrane region 113. On the silicon nitride film 103, thelower electrode 104 and extraction wiring 115, each composed of the sameconductive film, are formed. The lower electrode 104 is formed on thesilicon nitride film 103 covering the membrane region 113 and a vicinityregion thereof (a part of an external region of the membrane region113). The extraction wiring 115 is formed on the portion of the siliconnitride film 103 which is located outside the membrane region 113 to beconnected to the lower electrode 104.

Over each of the silicon nitride film 103, the lower electrode 104, andthe extraction wiring 115, a silicon oxide film 105 and a siliconnitride film 106 are formed successively. The vibrating film 112 isconstituted herein by the lower electrode 104 composed of the conductivefilm and the respective portions of the silicon nitride film 103, thesilicon oxide film 105, and the silicon nitride film 106 which arelocated in the membrane region 113. The vibrating film 112 is alsoformed with a plurality of leak holes 107 each connecting to the air gap109. Each of the silicon nitride films 103 and 106 is formed to coverthe entire surfaces of the lower electrode 104 and the silicon oxidefilm 105 including the inner wall surfaces of the leak holes 107. Thesilicon oxide film 105 is an electret film having a charge accumulatedtherein. Specifically, the charge is injected in the silicon oxide film105 by exposing the silicon oxide film 105 during a corona discharge orplasma discharge so that the silicon oxide film 105 electretized therebyis formed successfully. At this time, the silicon oxide film 105 may beeither exposed or covered with the silicon nitride films 103 and 106during the corona discharge or plasma discharge.

As shown in FIG. 3, the fixed film 110 composed of the conductive film118 covered with a lower-layer silicon nitride film 114 and anupper-layer silicon nitride film 119 is further formed above thevibrating film 112, i.e., above the silicon nitride film 106. The airgap 109 is formed between the vibrating film 112 and the fixed film 110in the membrane region 113 and the vicinity region thereof (a part ofthe external region of the membrane region 113), while a silicon oxidefilm 108 is formed between the silicon nitride film 106 or the siliconoxide film 102 and the fixed film 110 in the other region. In otherwords, the air gap 109 is formed over a region including at least theentire membrane region 113, while the fixed film 110 is supported abovethe vibrating film 112 by the silicon oxide film 108.

The fixed film 110 located above the air gap 109 is formed with aplurality of acoustic holes 111 each connecting to the air gap 109. Anopening 116 is provided in the fixed film 110 including the siliconnitride film 114 and in the silicon oxide film 108 to partially exposethe extraction wiring 115. The lower electrode 104 is electricallyconnected to the gate of the FET portion 20 shown in FIG. 2 via theextraction wiring 115. In addition, an opening 117 is provided in thesilicon nitride film 119 composing the fixed film 110 and the conductivefilm 118 composing the fixed film 110 is exposed therein such that theconductive film 118 is electrically connected thereby to the GNDterminal 25 of FIG. 2.

FIG. 4 is a plan view of the lower electrode 104 of the electretcondenser according to the present embodiment and the extraction wiring115 thereof. As stated previously, each of the lower electrode 104 andthe extraction wiring 115 is composed of the same conductive film. Asshown in FIG. 4, the lower electrode 104 is formed inside the membraneregion 113 and the plurality of leak holes 107 are formed in theperipheral portion of the lower electrode 104. The extraction wiring 115is formed to electrically connect the lower electrode 104 to theoutside.

A description will be given herein below to the reason that the lowerelectrode 104 is formed inside the membrane region 113. The capacitanceof the condenser in the ECM is determined by a capacitance componentwhich varies with the vibration of the vibrating film and by acapacitance component which does not vary with the vibration of thevibrating film. When a parasitic capacitance increases, the capacitancecomponent which does not vary with the vibration of the vibrating filmincreases disadvantageously so that the performance of the ECM isgreatly influenced thereby. To prevent this, the present embodiment hasprovided the lower electrode 104 of the electric condenser inside themembrane region 113. Since the arrangement eliminates the overlappingregion between the lower electrode 104 and the semiconductor substrate101, it is possible to eliminate a large-area MOS (metal oxidesemiconductor) capacitance composed of the lower electrode 104, thesilicon oxide film 102, and the semiconductor substrate 101. Morespecifically, the parasitic capacitance can be limited only to asmall-area MOS capacitance composed of the extraction wiring 115, thesilicon oxide film 102, and the semiconductor substrate 101. As aresult, an increase in the capacitance component (parasitic capacitance)which does not vary in the condenser can be prevented and therefore asmall-size and high-performance condenser can be implemented.

Of the components of the vibrating film 112 according to the presentembodiment, i.e., of the silicon nitride film 103, the lower electrode104 composed of the conductive film, the silicon oxide film 105, and thesilicon nitride film 106, the silicon nitride film 103, the siliconoxide film 105, and the silicon nitride film 106 each formed to coverthe membrane region 113 are formed to overlap the semiconductorsubstrate 101. In other words, the respective end portions of thesilicon nitride film 103, the silicon oxide film 105, and the siliconnitride film 106 are located above the semiconductor substrate 101. Onthe other hand, the lower electrode 104 of the vibrating film 112, whichis composed of the conductive film, is formed inside the membrane region113 not to overlap the semiconductor substrate 101. In other words, theend portion of the lower electrode 104 is located inside the membraneregion 113. The arrangement allows the resonant frequency characteristicof the vibrating film 112 to be controlled by adjusting the filmthickness of each of the silicon nitride film 103, the silicon oxidefilm 105, and the silicon nitride film 106. Thus, by allowing easycontrol of the capacitance component which varies under a pressure fromthe outside of the condenser, a small-size and high-sensitivity electretcondenser can be implemented.

A description will be given herein below to the reason that the siliconnitride films 103 and 106 are formed to cover the lower electrode 104and the silicon oxide film 105. When the electret composed of thesilicon oxide film comes in contact with a liquid, the charge in theelectret is significantly reduced. To suppress the reduction in thecharge of the electret, the present embodiment has covered at least thesurfaces (upper, lower, and side surfaces) of the silicon oxide film 105serving as the electret with the silicon nitride films 103 and 106. Morespecifically, the inner wall surfaces of the leak holes 107 are alsocovered completely with the silicon nitride film 106 such that thesilicon oxide film (electret) 105 is not exposed in each of the leakholes 107 formed in the vibrating film 112. As a result, it becomespossible to implement an electret condenser having an electret which isexcellent in moisture resistance and heat resistance.

FIG. 5 is a plan view of the silicon nitride film 114 composing thefixed film 110 of the electret condenser according to the presentembodiment. As described above, the plurality of acoustic holes 111 areformed in the fixed film 110 formed above the semiconductor substrate101 including the membrane region 113. Each of the acoustic holes 111 islocated in the membrane region 113 and the vicinity region thereof (apart of the external region of the membrane region 113).

A description will be given herein below to the operation of theelectret condenser according to the present embodiment. In the electretcondenser according to the present embodiment shown in FIG. 3, when thevibrating film 112 receives a sound pressure from above through theacoustic holes 111 and the air gap 109, it mechanically vibrates upwardand downward in response to the sound pressure. The electret condenseraccording to the present embodiment has a parallel-plate condenserstructure using, as the electrodes, the lower electrode 104 composingthe vibrating film 112 and the conductor film 118 composing the fixedfilm 110. Accordingly, when the vibrating film 112 vibrates, thedistance between the lower electrode 104 and the conductive film 118 asthe electrodes changes to change the capacitance (C) of the condenser.Since the charge (Q) accumulated in the condenser is constant, thechange in the capacitance (C) of the condenser causes a change in thevoltage (V) between the lower electrode 104 and the fixed film 110(conductive film 118). The reason for this is that the condition givenby the following numerical expression (1) should be physicallysatisfied.Q=C·V  (1)

In addition, when the voltage (V) between the lower electrode 104 andthe fixed film 110 (conductive film 118) changes, the gate potential ofthe FET portion 20 also changes because the lower electrode 104 iselectrically connected to the gate of the FET portion 20 of FIG. 2.Thus, the vibration of the vibrating film 112 changes the gate potentialof the FET portion 20 so that the change in the gate potential of theFET portion 20 is outputted as a voltage change to the external outputterminal 29 of FIG. 2.

As described above, the present embodiment allows protection of thecharged silicon oxide film 105 with the silicon nitride films 103 and106. Specifically, by covering the surfaces of the silicon oxide film105 made of a material showing remarkable absorption of atmosphericmoisture or the like with the silicon nitride films 103 and 106 toprevent the silicon oxide film 105 from being exposed to an ambientatmosphere, it becomes possible to suppress a reduction in the amount ofcharge in the silicon oxide film 105. This allows an improvement in thereliability of the electret. As a result, an electret condenser havingan electret structure which is excellent in moisture resistance, such asan ECM, can be provided. By producing such an ECM by using a MEMStechnology, a small-size ECM which does not require a charge supplycircuit can be provided.

Thus, the present embodiment makes it possible to implement ahigh-reliability, small-size, and high-performance microphone and alsowidely supply various practical devices each equipped with themicrophone to a society.

Although the present embodiment has covered the lower surface of thecharged silicon oxide film 105 with the silicon nitride film 103 withthe lower electrode 104 interposed therebetween, the lower surface ofthe silicon oxide film 105 may also be covered directly with the siliconnitride film.

Although the present embodiment has covered the surface of the chargedsilicon oxide film 105 with the silicon nitride film, the surface of thecharged silicon oxide film 105 may also be covered with an insulatingfilm of another type having a higher moisture resistance than thesilicon oxide film instead of the silicon nitride film.

Alternatively, the present embodiment may also use silicon orpolysilicon doped with an impurity, gold, a refractory metal, aluminum,an aluminum-containing alloy, or the like as a conductor materialcomposing the lower electrode 104.

Alternatively, the present embodiment may also use silicon orpolysilicon doped with an impurity, gold, a refractory metal, aluminum,an aluminum-containing alloy, or the like as the material of theconductive film 118 composing the fixed film 110.

In the present embodiment, a substrate made of an insulating materialmay also be used instead of the semiconductor substrate 101.

INDUSTRIAL APPLICABILITY

The present invention relates to an electret condenser having avibrating electrode and a fixed electrode. When applied to an ECM formedby using a MEMS technology or the like, the present invention canparticularly improve the performance and reliability of the ECM and istherefore extremely useful.

1. An electret comprising: a charged silicon oxide film; a firstinsulating film formed to cover upper and side surfaces of the siliconoxide film; and a second insulating film formed to cover a lower surfaceof the silicon oxide film, wherein at least one of the first insulatingfilm and the second insulating film is formed to be in contact with atleast one of the upper, side, and lower surfaces of the silicon oxidefilm.
 2. The electret of claim 1, wherein each of the first and secondinsulating films is a silicon nitride film.
 3. The electret of claim 1,wherein the silicon oxide film has been charged by a plasma discharge ora corona discharge.
 4. The electret of claim 1, wherein the firstinsulating film is formed to be in contact with the upper and sidesurfaces of the silicon oxide film.
 5. The electret of claim 4, whereinthe second insulating film is formed to be in contact with the lowersurface of the silicon oxide film.
 6. The electret of claim 1, whereinthe second insulating film is formed to be in contact with the lowersurface of the silicon oxide film.
 7. An electret condenser comprising:a fixed film having a first electrode; and a vibrating film disposedwith an air gap interposed between itself and the fixed film, whereinthe vibrating film has a multilayer structure composed of a chargedsilicon oxide film, a second electrode, a first insulating film, and asecond insulating film, the silicon oxide film is disposed between thefirst and second electrodes, upper and side surfaces of the siliconoxide film are covered with the first insulating film, a lower surfaceof the silicon oxide film is covered with the second insulating film,and at least one of the first insulating film and the second insulatingfilm is formed to be in contact with at least one of the upper, side,and lower surfaces of the silicon oxide film.
 8. The electret condenserof claim 7, wherein the lower surface of the silicon oxide film iscovered with the second insulating film with the second electrodeinterposed therebetween.
 9. The electret condenser of claim 7, whereinthe vibrating film is formed with a plurality of through holes eachreaching the air gap and a surface of the silicon oxide film which formseach of respective inner wall surfaces of the plurality of through holesis covered with the first insulating film.
 10. The electret condenser ofclaim 7, wherein each of the first and second insulating films is asilicon nitride film.
 11. The electret condenser of claim 7, whereineach of the first and second electrodes is made of aluminum, an aluminumalloy, silicon, polysilicon, gold, or a refractory metal.
 12. Theelectret condenser of claim 7, wherein an area of the second electrodeis smaller than an area of the silicon oxide film.
 13. The electretcondenser of claim 7, wherein the silicon oxide film has been charged bya plasma discharge or a corona discharge.
 14. The electret condenser ofclaim 7, wherein the first insulating film is formed to be in contactwith the upper and side surfaces of the silicon oxide film.
 15. Theelectret condenser of claim 14, wherein the second insulating film isformed to be in contact with the lower surface of the silicon oxidefilm.
 16. The electret condenser of claim 7, wherein the secondinsulating film is formed to be in contact with the lower surface of thesilicon oxide film.
 17. An electret condenser comprising: asemiconductor substrate having a region removed to leave a peripheralportion thereof; and a vibrating film formed on the semiconductorsubstrate to cover the region, wherein the vibrating film has amultilayer structure composed of a charged silicon oxide film, anelectrode film, a first insulating film, and a second insulating film,upper and side surfaces of the silicon oxide film are covered with thefirst insulating film a lower surface of the silicon oxide film iscovered with the second insulating film, and at least one of the firstinsulating film and the second insulating film is formed to be incontact with at least one of the upper, side, and lower surfaces of thesilicon oxide film.
 18. The electret condenser of claim 17, wherein thelower surface of the silicon oxide film is covered with the secondinsulating film with the electrode film interposed therebetween.
 19. Theelectret condenser of claim 17, wherein the electrode film is disposedbetween the semiconductor substrate and the silicon oxide film.
 20. Theelectret condenser of claim 17, wherein each of the first and secondinsulating films is a silicon nitride film.
 21. The electret condenserof claim 17, wherein the electrode film is formed inside the region innon-overlapping relation with the semiconductor substrate.
 22. Theelectret condenser of claim 17, wherein the first insulating film isformed to be in contact with the upper and side surfaces of the siliconoxide film.
 23. The electret condenser of claim 22, wherein the secondinsulating film is formed to be in contact with the lower surface of thesilicon oxide film.
 24. The electret condenser of claim 17, wherein thesecond insulating film is formed to be in contact with the lower surfaceof the silicon oxide film.
 25. An electret comprising: a charged siliconoxide film; a first silicon nitride film formed to cover upper and sidesurfaces of the charged silicon oxide film; and a second silicon nitridefilm formed to cover a lower surface of the charged silicon oxide film.26. The electret of claim 25, wherein the first silicon nitride film isformed to be in contact with the upper and side surfaces of the siliconoxide film.
 27. The electret of claim 26, wherein the second siliconnitride film is formed to be in contact with the lower surface of thesilicon oxide film.
 28. The electret of claim 25, wherein the secondsilicon nitride film is formed to be in contact with the lower surfaceof the silicon oxide film.